In recent years, numerous electronic appliances that transmit power in a non-contact manner have been developed. In order to transmit power to an electronic appliance in a non-contact manner, a power transmission system is often adopted that employs a magnetic field coupling scheme in which a coil module is included in both a power transmission unit and power reception unit.
However, in such magnetic field coupling scheme, the amount of magnetic flux passing through each coil module is greatly affected by the electromotive power and, in order to transmit power with high efficiency, high accuracy is needed in the control of the relative positions in a planar direction of the coil of the coil module on the power transmission unit side (primary side) and the coil of the coil module on the power reception unit side (secondary side). In addition, since coil modules are used as coupling electrodes, it is difficult to reduce the size of the power transmission unit and the power reception unit. Moreover, in electronic appliances such as portable appliances, there have also been issues in that it has been necessary to consider the effect that heat generated by the coil will have on the secondary battery and there is a risk that there will be a bottleneck from the viewpoint of layout design.
Accordingly, for example, power transmission systems have been developed that employ an electrostatic field. In Patent Document 1, an energy conveyance system is disclosed in which high power transmission efficiency is realized by forming a strong electric field between a coupling electrode of a power transmission unit and a coupling electrode of a power reception unit. FIG. 10 is a schematic diagram that illustrates the configuration of a power transmission system of the related art. As illustrated in FIG. 10, the power transmission system of the related art includes a large passive electrode 3 and a small active electrode 4 on a power transmission unit (power transmission device) 1 side and includes a large passive electrode 5 and a small active electrode 6 on a power reception unit (power reception device) 2 side. The active electrode 4 on the power transmission unit 1 side and the active electrode 6 on the power reception unit 2 side face each other and a strong electric field 7 is formed therebetween, whereby high power transmission efficiency is realized.
In addition, in Patent Documents 2 and 3, a method of detecting a foreign substance on the basis of changes in transmitted power in an magnetic field coupling scheme power transmission system in which a coil module is provided in both a power transmission unit (power transmission device) and a power reception unit (power reception device) is disclosed and in Patent Document 3 a power transmission control flow is disclosed in which communication is performed between a power transmission unit and a power reception unit.
Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT application) No. 2009-531009
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2010-213414
Patent Document 3: Japanese Patent No. 4725664
In the power transmission system of the related art, changes in the voltage at the electrode are monitored by the power transmission unit and in the case where an abnormal change occurs, transmission control to stop transmission of power is generally performed. However, there has been a problem when transmitting power using an electric field coupling scheme in that it is difficult to ensure safety with respect the high electric field generated between the electrodes.
In addition, there has been another problem in that standby power consumption is large even when no power reception unit is mounted on the power transmission unit due to voltage changes at the electrode being monitored by the power transmission unit.